1. Field of the Invention
The present invention relates to a silencer, and more particularly, to a silencer wherein the diameter and the length of a pipe are adjusted to improve noise reduction characteristics of the silencer.
2. Discussion of the Related Art
Generally, a silencer is an apparatus for reducing noise generated from an internal-combustion engine or ventilation equipment. Based on its operating principle, the silencer may be classified as an expansion type silencer constructed in a structure in which noise is diffused from a small-diameter pipe to a large-diameter pipe so as to reduce the noise, a resonance type silencer constructed in a structure in which noise is diffused from a small-diameter pipe to a wide resonance chamber through a plurality of holes formed in the small-diameter pipe so as to offset the noise, or an absorption type silencer constructed in a structure in which the silencer is provided with a noise-absorbing material, by which noise is absorbed, and therefore, the noise is reduced.
The structure and the operating principle of a conventional expansion type silencer will be described with reference to FIG. 1.
The conventional expansion type silencer includes a first cylindrical connection pipe 10, an cylindrical expansion pipe 50 communicating with the first connection pipe 10 at one end thereof, the expansion pipe 50 having a sectional area greater than that of the first connection pipe 10, and a second cylindrical connection pipe 30 communicating with the other end of the expansion pipe 50.
The noise reduction efficiency of the silencer may be represented by various values. Typically, noise reduction efficiency of the silencer may be represented by transmission loss (TL) which algebraically expresses the ratio of input sound pressure level to transmission sound pressure level when a fluid is transmitted through the silencer.
When the first connection pipe 10 and the second connection pipe 30 have the same sectional area, the transmission loss (TL) of the expansion type silencer as shown in FIG. 1 may be represented by the following equation:
                    TL        =                  10          ⁢                      log            ⁡                          (                              1                +                                                      1                    4                                    ⁢                                                            (                                              m                        -                                                  1                          m                                                                    )                                        2                                    ⁢                                      sin                    2                                    ⁢                  kL                                            )                                                          [                  Equation          ⁢                                          ⁢          1                ]            
Where, m is the ratio in sectional area of the first connection pipe to the expansion pipe, k is wave number, and L is the length of the expansion pipe.
On the other hand, the connection regions between the expansion pipe 50 and the first and second connection pipes are generally designed such that the sectional area of the connection regions is not abruptly changed, i.e., gradually changed, so as to minimize flow resistance. When the shape of the connection regions is changed, the acoustic characteristics of the silencer are also changed. Strictly speaking, the symbol L in the above equation is not the actually measured length of the expansion pipe 50 but the length acoustically converted by adding and subtracting the actually measured length of the expansion pipe 50.
Specifically, the symbol L in Equation 1 means the acoustic length corrected from the actually measured length according to the concretely applied shape of the expansion pipe 50. Here, a method of calculating the acoustic length is well known to those skilled in the art to which the present invention pertains, and therefore, a detailed description thereof will not be given.
As shown in FIG. 1, not the distance between opposite ends of the actual expansion pipe 50 but the distance between points extending from the opposite ends of the expansion pipe 50 to specific positions of the respective connection regions is indicated by L.
In Equation 1, TL is maximized when sin2kL is 1. Consequently, the transmission loss (TL) is maximized when the following condition is satisfied:
                              kL          =                                    n              ⁢                                                          ⁢              π                        2                          ,                  n          =          1                ,        3        ,        5        ,        …                            [                  Equation          ⁢                                          ⁢          2                ]            
In the above equation, the relation of
  k  =            2      ⁢      π        λ  λ is the wavelength of an input sound source) is satisfied, and therefore, the above equation may be expressed by the following equation:
                              L          =                                    λ              4                        ⁢            n                          ,                  n          =          1                ,        3        ,        5        ,        …                            [                  Equation          ⁢                                          ⁢          3                ]            
As can be seen from the above equation, the transmission loss (TL) of the silencer is maximized when the length of the expansion pipe 50 is odd multiples of λ/4.
Consequently, when designing the silencer, the wavelength λ at a target frequency band is calculated such that the maximum noise reduction efficiency can be exhibited at the target frequency band in a specific operation condition, and the length L of the expansion pipe 50 is decided based on Equation 3.
However, the conventional silencer with the above-stated construction has a problem in that the target frequency band noise cut off as a fluid passes through the silencer is reincreased due to a resonance mode of a pipe connected to the silencer.
In addition, the conventional silencer has a problem in that, when an additional silencing unit is mounted in the silencer, the dimensions of the expansion pipe designed according to the target frequency band become insignificant.